The invention relates to a printhead suitable for use with image forming systems, and more particularly relates to the utilization of one or more electrodes for blocking electric field transmission to prevent plasma generation in predetermined locations within the printhead.
Current image forming systems utilize different printhead technologies to form desired printed images. Some of the printhead technologies include a process of charging a surface of an image-receiving member, such as a dielectric drum, with a latent charge image. The term image-receiving member includes any suitable structure supporting the latent image of charge, and can include a dielectric or photoconductive drum, a flat or curved dielectric surface, or a flexible dielectric belt, which moves along a predetermined path. The image-receiving member can also comprise a liquid crystal, phosphor screen, or similar display panel in which the latent charge image converts into a visible image. The image receiving member typically includes on an exterior surface a material such as a dielectric or photoconductor that lends itself to receiving the latent charge image. A number of organic and inorganic materials are suitable for the dielectric layer of the image receiving member. The suitable materials include glass enamel, anodized and flame or plasma sprayed high-density aluminum oxide, and plastic, including polyamides, nylons, and other tough thermoplastic or thermoset resins, among other materials.
The image receiving member moves past an image forming device, such as a printhead, which produces streams of accelerated electrons as primary charge carriers. The electrons reach the drum, landing in the form of a latent charge image. The latent charge image then receives a developer material, to develop the image, and the image is then transferred and fused to a medium, such as a sheet of paper, to form a printed document.
The printhead most often includes layers having a multi-electrode structures that define an array of charge generation sites. Each of the charge generation sites, when the electrodes are actuated, generates and directs toward the drum a stream of charge carriers, e.g., electrons, to form a pointwise accumulation of charge on the drum that constitutes the latent image. A representative printhead generally includes a first collection of drive electrodes, e.g., RF-line electrodes, oriented in a first direction across the direction of printing. A second collection of control electrodes, e.g., finger electrodes, oriented transversely to the drive electrodes, forms spatially separated cross points with the first collection of drive electrodes. In the cross points, electrodes form charge generating sites at which charges originate. A dielectric layer couples to, and physically and electrically separates and insulates, the RF-line electrodes from the finger electrodes.
The printhead can also include a second dielectric or insulating layer and a third electrode structure, often identified as a screen electrode. The second dielectric/insulating layer couples to the finger electrodes and the screen electrodes. The screen electrode, usually in the form of a conductive sheet, has a plurality of apertures aligned with the charge generation sites to allow the stream of charge carriers to pass through. The polarity of the charge carriers passing through the apertures depends on the voltage difference applied to the finger and screen electrodes. The polarity of the charged particles accumulated on the drum to create latent images is determined by the voltage difference between the screen electrode and the drum surface. The charged particles of appropriate polarity are inhibited from passing through the aperture, depending upon the sign of their charge, so that the printhead emits either positive or negative charge carriers, depending on its electrode operating potentials.
In some instances, it is desirable to prevent the creation of plasma, and thus, the generation of charged particles in certain places that have not been properly sealed due to structural or systematic constraints. Typically, places where undesired plasma can eventually arise are the gaps between the finger electrodes in the cross points with the RF-line electrodes. Such places are usually sealed by a dielectric that is simultaneously used as a spacer layer between the screen and the finger electrodes. In printheads suitable for high resolution print, and especially for printheads with a low number of RF-electrodes, sealing of the gaps between the finger electrodes by the dielectric spacer layer can be difficult. In addition, in such printheads with a high density of finger electrodes, the dielectric spacer interacts with the plasma resulting from the charge generation sites. In the typical case of spacers made of an organic material, the interaction with plasma results in degradation of the charge generation capability, and therefore in degradation of the print quality and in shortening of the printhead life.
There exists in the art a need for a printhead that does not require the use of dielectric layers to suppress the plasma formation in predetermined locations along the finger electrodes. The present invention is directed toward such a solution.
A printhead, in accordance with one example embodiment of the present invention, has at least a first electrode layer (e.g., RF-line electrodes) and at least a second electrode layer (e.g., finger electrodes). Electrodes of both layers are electrically insulated with respect to each other by a dielectric material. There is, in addition, a plurality of plasma suppressing electrodes arranged within the dielectric material to hinder plasma generation at predetermined locations.
The present invention further provides for a printhead having a first electrode layer and a second electrode layer that are electrically insulated from each other by a dielectric material and a plurality of plasma suppressing electrodes disposed exterior to one of the electrode layers. An additional dielectric segment is located between the electrode layer and the plasma suppressing electrodes.